On Fri, Jul 09, 2021 at 11:19:15AM +0200, Christian Brauner wrote: > On Thu, Jul 08, 2021 at 01:57:38PM -0400, Vivek Goyal wrote: > > Currently user.* xattr are not allowed on symlink and special files. > > > > man xattr and recent discussion suggested that primary reason for this > > restriction is how file permissions for symlinks and special files > > are little different from regular files and directories. > > > > For symlinks, they are world readable/writable and if user xattr were > > to be permitted, it will allow unpriviliged users to dump a huge amount > > of user.* xattrs on symlinks without any control. > > > > For special files, permissions typically control capability to read/write > > from devices (and not necessarily from filesystem). So if a user can > > write to device (/dev/null), does not necessarily mean it should be allowed > > to write large number of user.* xattrs on the filesystem device node is > > residing in. > > > > This patch proposes to relax the restrictions a bit and allow file owner > > or priviliged user (CAP_FOWNER), to be able to read/write user.* xattrs > > on symlink and special files. > > > > virtiofs daemon has a need to store user.* xatrrs on all the files > > (including symlinks and special files), and currently that fails. This > > patch should help. > > > > Link: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@xxxxxxxxxx/ > > Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx> > > --- > > Seems reasonable and useful. > Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx> > > One question, do all filesystem supporting xattrs deal with setting them > on symlinks/device files correctly? Wrote a simple bash script to do setfattr/getfattr user.foo xattr on symlink and device node on ext4, xfs and btrfs and it works fine. https://github.com/rhvgoyal/misc/blob/master/generic-programs/user-xattr-special-files.sh I probably can add some more filesystems to test. Thanks Vivek > > > fs/xattr.c | 10 ++++++---- > > 1 file changed, 6 insertions(+), 4 deletions(-) > > > > diff --git a/fs/xattr.c b/fs/xattr.c > > index 5c8c5175b385..2f1855c8b620 100644 > > --- a/fs/xattr.c > > +++ b/fs/xattr.c > > @@ -120,12 +120,14 @@ xattr_permission(struct user_namespace *mnt_userns, struct inode *inode, > > } > > > > /* > > - * In the user.* namespace, only regular files and directories can have > > - * extended attributes. For sticky directories, only the owner and > > - * privileged users can write attributes. > > + * In the user.* namespace, for symlinks and special files, only > > + * the owner and priviliged users can read/write attributes. > > + * For sticky directories, only the owner and privileged users can > > + * write attributes. > > */ > > if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) { > > - if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode)) > > + if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode) && > > + !inode_owner_or_capable(mnt_userns, inode)) > > return (mask & MAY_WRITE) ? -EPERM : -ENODATA; > > if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) && > > (mask & MAY_WRITE) && > > -- > > 2.25.4 > > >
